Tutorial: Build an E-Commerce App

In this tutorial, you'll build an event-sourced e-commerce application from scratch. By the end, you'll have a working system with products, orders, customers, and reactive read models.

What You'll Build

• Product aggregate — create products, manage prices and stock
• Order aggregate — place orders with line items, cancel orders
• Customer aggregate — manage customer profiles and addresses
• Product Catalog projection — a read model that stays in sync with product events

What You'll Learn

Chapter	Concept
1. Project Setup	Go module, in-memory backends, bootstrap
2. Your First Aggregate	aggregate.Base, events, appliers
3. Events & State	Multiple events, typed event data
4. Codec Registry	Event serialization
5. Repositories	Saving and fetching aggregates
6. Commands	Command bus, dispatching, handlers
7. The Order	Second aggregate, validation patterns
8. The Customer	Value objects, addresses
9. Aggregate Splitting	Splitting aggregates, shared UUIDs
10. Projections	Read models, schedules
11. Production Backends	MongoDB, NATS, PostgreSQL
12. Testing	Testing aggregates and projections

Prerequisites

• Basic Go knowledge (structs, interfaces, generics)
• Familiarity with event sourcing and DDD concepts (see Introduction)

The Colocation Pattern

Throughout this tutorial, we keep everything related to an aggregate in a single file. Event names, event data types, commands, the aggregate struct, business methods, and event appliers all live together. This makes aggregates self-contained and easy to understand.

shop/
  main.go           # Bootstrap, wiring
  product.go        # Product aggregate + events + commands
  pricing.go        # Pricing aggregate (split from Product)
  order.go          # Order aggregate + events + commands
  customer.go       # Customer aggregate + events + commands
  catalog.go        # Product catalog projection

Let's get started.